The Greeks arrived and took a lot with them. Central Asia’s depictions of Buddha have been realistic (other than the fact that no one really knows what Buddha looked like) since the 1st c AD, which is not as far back as this, but these giant statues that the Chinese reported as suddenly appearing apparently no longer exist, and may represent the kind of monumental depictions such as were done in the Middle Ages.

The Mauryan Empire ruled a chunk of what is now Afghanistan, and the third ruler converted to Jainism (his grandfather and founder of the dynasty had been Jainist) which is known to have built large images, that’s a possibility as well.

Classical Greeks are also known to have built at least two monumental statues — one was Zeus in the “seven wonders” temple, and the other the Colossus of Rhodes. Maybe there were more, but none come to mind. They did lifesize or somewhat larger than life size images, and pimped them up with paint, shells, and other colorful doodads.